Vietnam coffee export prices slipped to discounts against the international benchmark as robusta prices in London rose last week on supply worries, while ample harvests kept Indonesian coffee price rebates steady, traders said on Thursday.

Vietnam's 5-percent black and broken grade 2 robusta prices were at $10-$20 discounts per tonne to London's ICE September contract, from premiums of $10-$20 the week before as London futures hit two-month highs on tightening supply prospects.

London prices have since lost some ground, and were trading at $2,070 per tonne as of 0949 GMT on Thursday.

Vietnamese coffee exporters, faced with dwindling robusta supplies, are paying more to buy beans from international trading houses who scooped up much of the crop early in the season.

The country's coffee exports in May fell 9.4 percent compared with April.

Coffee exports are expected to touch 90,000-120,000 tonnes in June, traders said.

Farmers in Vietnam, the world's biggest robusta producer, quoted local prices at 44,500 dong-45,000 dong ($1.96-$1.98) per kg, marginally down from 45,000-45,100 dong last week.

"Farmers are holding on to their last coffee beans as prices of other crops such as pepper or passion fruit are falling," said a Ho Chi Minh-city based trader.

In rival Indonesia, robusta grade 4, 80 defects was quoted at a discount of $30 per tonne to the London July contract, unchanged from last week, amid ample supply following weeks of harvesting, said traders in Lampung, Indonesia's main coffee growing area.

There were fewer buyers though, they said, as the country heads to a long holiday period from June 23-30.